Determination of tetrahydrocarbon lead impurities in gases



Jan. l, 1963 LJ. sNYDER ETAL 3,071,446

(U qwfous Filed Feb. 23, 1962 United States Patent ce Patented Jan. l, 1963 DETERMINATION F TETRAHYBRCARBQN )LEAD EMPURETIES EN GASES Louis J. Snyder and Samuel R. Henderson, Baton Rouge,

La., assignors to Ethyl Corporation, New York, N.Y., a corporation or Delaware Feb. 23, i962, Ser. No. 175,002

l tliaim. (Cl. 23-232) This invention relates to the determination of organometallic compounds in extremely low or trace concentrations in the vapor phase, in gaseous fluids. More particularly, preferred forms of the invention lare directed to an improvement in a sampling and analytical process for the determination of minute quantities of lead compounds, particularly alkyllead compounds, in air or in gaseous atmospheres.

It has long been desirable and necessary that a precise and accurate method be available to determine the concentration of lead or lead compounds in gaseous iiuids or atmospheres. In particular, such a procedure has been essential for the determination of extremely minute concentrations of such materials in the lead working industries, or in chemical plants or other installations wherein lead and lead compounds are fabricated, processed, stored, etc. Because of the toxicological properties yof lead and lead compounds methods having been necessary for the determination of extremely low concentrations of such materials. For example, it is frequently necessary to determine the concentration of these materials in a range of 0 to l()v or l5 micrograms per cubic foot of the gaseous atmosphere. A microgram being l-millionth of a gram, it is seen that the analytical requirements includeI a high degree of sensitivity and accuracy in the sampling and analysis of systems of this character.

A generally recognized analytical procedure for the determination of lead compounds in gases is the Snyder et al. procedure (Analytical Chemistry 20, 772 (1948)). According to said procedure, a specimen of gas is contacted with an aqueous solution of iodine. This results in the "iixation of the lead or lead compound in the sample as an iodide of lead.

The resultant solution is then treated with an `aqueous reducing reagent including sodium sulte, ammonium citrate and potassium cyanide. The function of this solution is to convert the elemental iodine to a reduced or compound form, viz., an alkali metal iodide in solution. The ammonium citrate prevents the precipitation of hydroxides of other metal impurities possibly present and the cyanide content prevents interference in subsequent analytical steps, by other metal compounds. Upon cornpletion of this operation, then, the system consists of an aqueous solution containing the lead as lead diiodide, Pblz, in aqueous phase. The specimen is then contacted with a solution of dithizone (diphenylthiocarbazone) which forms lead dithizonate, which is soluble in an organic solvent. The organic phase, then, dissolves the lead dithizonate and is stratied out of and below the aqueous phase. The amount of the lead is determined according to the intensity of color, by comparison with previously prepared samples of known concentrations, or color discs of some standard material. Lead dithizonate has a clearly perceptible red color. Thus, the quantity of lead can be correctly determined and is usually reported in terms of micrograms per cubic foot of the gas which was the original sample.

The foregoing procedure is quite operable for substantially all the known lead compounds and particularly the organolead compounds which it is desired to be able to determine in the atmosphere of tanks or of various industrial operations. However, in the case of the lead being present as an organolead compound other than tetraethyllead, it is found that particular diiculties arise which are illustrated more specifically hereinafter. These dimculties are, primarily, that a particularly long contact time was necessary in the very first step, viz., contacting the gas sample with an aqueous iodine solution. Thus, scrubbing etiiciencies or absorption efliciencies of the order of only 27 percent were achieved, in the case of determination of the quantity of, for example, tetrameth'yllead, whereas in the case of tetraethyllead being the lead compound, a comparable contacting time provided an absorption of percent. This deficiency could be partly overcome by drastically reducing the rate of sampling'and of contact of the gas sample with the potassium iodide solution, but this obviously severely limits the efficiency and applicability of the technique. Accordingly, a severe need has existed for a new and improved method whereby rapid sampling, and extraction of lead compounds from the gas sample and rapid subsequent quantitat-ive determination of the quantity of such lead could be readily achieved.

It is therefore an object of this invention to provide a new and improved sampling and analytical method. More particularly, an object of the invention is to provide an improvement in the dithizone method of lead determination, which improvement provides and accomplishes rapid and complete removal of lead compounds from the gas sample and facilitates accurate and rapid determination of the quantity of said lead compounds. Another object is to provide an improvement in sampling and analytical procedure which greatly facilitates the iield determination of the quantity of lead compounds in gaseous atmospheres. A specific object is to provide an improved sampling-reacting method, which does not require a liquid scrubbing device for contacting with the gaseous specimen. Other objects will appear hereinafter.

The present invention, then, consists of an improvement in the Snyder dithizone method of determination Iand analysis of very small amounts of lead in gaseous fluids. The improvement is based upon the discovery that contacting a gaseous sample with a bed of elemental iodine crystals results in substantially quantitative removal of lead components contained in the gas sample. The eliicient removal is not contingent upon the identity of the lead containing component which is to be determined. Alternatively stated, it is now discovered that the present improvement is equally effective for the'lixation of llead appearing Ias tetramethyllead, trimethyl lead halides, tetraethyllead, and other forms of lead, including particulate lead or dust particles, whereas the prior method was highly eiiective only for tetraethyllead as the lead containing contaminant.

In the most highly refined forms of the invention, a further improvement in the procedure is incorporated, Viz., the use of a multi-component organic reagent including a chlorohydrocarbon constituent and an alcohol constituent. Said solution facilitates the isolation of lead dithizonate in a concentration permissive of direct determination of the analytical answer by color comparison with standard colored glass discs or other reference color means such aS standard solutions.

The details of the present invention and of the mode of operation and the variables involved therein will be fully understood from the following detailed description and from the accompanying figures wherein FIG. l is a schematic representation of the entire sampling and analytical sequence, showing the present improvement. FIG. 2 is an illustration of a particular apparatus employed for the present improvement.

Referring to FIG. l, -this is a schematic diagram indicating the full sequence of operations in the entire sampling and analytical procedure. The iirst step, comprising the present improvement, is the passage or aspiration of a standard size sample of the gas through a contacting step. In this operation, the -gas is passed through a bed of elemental iodine crystals contained in a contacting chamber and usually atiixed in place by surrounding portions or layers of foraminous material, typically glass wool. The said contacting results in substantially complete re moval of the lead component by adsorption on and/or reaction with the iodine crystals.

The next step of operation is the completion of conversion of the lead component to lead iodide, Pblg, which is accomplished by contacting the iodine crystals, having the lead component aixed therein, with an aqueous solution of acidic alkali metal iodide, i.e. acidic potassium iodide. Suicient residence time is allowed to insure complete conversion of the lead to lead iodide-usually several minutes. Substantially all the elemental iodine is dissolved in the aqueous acidic potassium iodide solution. This step, in addition to converting the Ilead to lead iodide, removes the lead material from the chamber employed for collection and permits transmittal to analytical vessels for subsequent operations.

The next operation is treatment of the aqueous solution, containing lead iodide and elemental iodine, with an ammonical reducing reagent, having several additional components. A typical reducing reagent is anhydrous sodium suliite, in aqueous ammonia containing adequate concentrations of potassium cyanide and ammonium citrate therein. The reaction accomplished with this reagent is the conversion of dissolved elemental iodine by reduction to an alkali metal iodide. A small portion of the same reducing reagent is employed 'to dissolve a measured portion solid dithizone crystals, forming ammonium dithizonate solution. The said ammonium dithizonate solution, and a solvent is then added to the aqueous system resulting from the iodine reduction step above described. The solvent system in the present improved process consists of equal parts of methyl alcohol and a mixture of carbon tetrachloride and chloroform. As more fully hereinafter described, -this mixture of water soluble and water insoluble components in the solvent reagent particularly facilitates the operation by assuring a resultant concentration of lead dithizonate whereby the lead concentration in lthe original sample is directly determinable without laborious calculations.

The mixture of the ammonium dithizonate solution, the solvent solution, and the aqueous system is agitated vigorously and allowed to separate into two phases. The lower and more dense phase is the lead dithizonate dissolved in the mixture of chloroform and carbon tetrachloride. The upper layer is the aqueous phase containing the inorganic iodide (but no lead), the alcohol component from the solvent additive, and the other components or reagents added in the previous steps.

The last step in the procedure is the determination of the lead content by comparison of the organic phase, for color, with reference standards. These standards can be solutions of lead dithizonate in the same organic solvent, or calibrated colored glass discs. By lthe appropriate adjustment of proportions of sample `and reagent quantities as will be clear hereinafter, the said color comparison step admits of a determination which is expressed directly as micrograms of lead per cubic foot of the gas sample.

Various conventional or readily available lapparatus units can be employed in -the various steps of the overall analytical method. Some of the apparatus components are standard laboratory glassware items, or items of apparatus units which have been heretofore used in the implementation ofthe Synder dithizone method of analysis of lead in gas.

Among the several apparatus units usually employed in carrying out the analytical process is a pump or other device for aspirating the gas sample through the scrubbing chamber, wherein the gas is contacted with the crystalline, elemental iodine. Such a pump may be a handoperated reciprocating pump whereby a standard number of reciprocations delivers a known volume gas sample. Alternatively a positive displacement, power operated rotating pump can be employed. Still another unit for aspirating a gas sample is an aspirating tube, aspiration action being induced -by the evaporation of a finite calibrated quantity of some propellant, such as are used in spray container devices.

Another necessary apparatus component is a suitable device, including a chamber, for providing the contact zone for contacting the gas sample with elemental iodine. One such suitable contacting chamber or iodine scruhber is illustrated in FIG. 2 and is discussed hereinafter.

Additional apparatus units include tunnels, the usual laboratory glassware equipment for preparation of reagent solutions, when the reagent solutions are prepared, storage bottles, color comparator tubes, and suitable color comparator devices. In this last category is the Hellige comparator, available from Hellige, Inc., New York. With color disc No. 620S-10, such an apparatus is particularly suitable for tield determination, and employs, as indicated, a color disc having various zones tinted to the correct color intensity whereby a direct color comparison can be made between an organic lead dithizonate solution and the reference color, thereby determining by direct visual observation, the concentration of the lead in the gas sample.

As already mentioned, a necessary apparatus component is a suitable chamber having the iodine crystals employed for contacting the gas specimen, affixed or positioned therein. One such apparatus is illustrated by FIG. 2. Referring to FIG. 2, this is a cross sectional, elevation view of a suitable embodiment for such apparatus component, the main portion of the apparatus being a tube 11, usually of glass. A typical size thereof is approximately l0 millimeters internal diameter and a length of 21/2 to 3 inches. Internal shoulders 12, 13 are provided in the tube element 11 and these serve to retain in position the packed-in contents thereof. The contents include, in this embodiment a layer of fibrous glass 14, a layer of iodine crystals 15, and a second layer of iibrous glass 16. In normal use the incoming gas sample is passed through the thinner layer of glass tiber 14, then the bed of iodine crystals 15 and lastly through the thicker bed of ber glass 16. In a typical embodiment about one gram of iodine crystals, screened through a number 16 sieve and collected on a number 30 sieve, is employed. The total weight of the brous glass, approximately micron size, is about 0.2 gram and the relative depths of the two layers, 14 and 16, is 1:3.

The foregoing specific embodiment of apparatus hav ing the appropriate quantity of iodine and support material is particularly suitable for analytical operations wherein a one cubic foot gas sample is employed, and the concentration of the lead compounds in the sample would be in the range of up to 20 to 25 micrograms per cubic foot. It will be quite clear that, for larger samples, higher sampling rates, and other variations, that the proportions of the scrubber unit and the quantity of iodine employed can be appreciably varied according to circumstances.

The following working example illustrates the operation of the entire procedure with the present improvement.

In the following working example, the crystalline iodine specimen employed for the improved sampling and reaction step was as already described, one gram of iodine crystals (through a No. 16 sieve and retained on a No. 30 sieve), with the micron size, brous glass layers on each side of the layer of the iodine crystals.

The several reagent solutions employed inthe complete analysis were as identified below:

Acidic potassium iodide solution- The stock solution was made by dissolving 450 grams of potassium iodide, 650 ml. of water and approximately l ml. of concentrated ammonium hydroxide. Any trace amounts of lead contaminant introduced in the reagents as such can be removed, ifdeemed necessary, by treating =vvith di-beta naphthyl thiocarbazone in chloroform. Ordinarily, deleading is not necessary. Portions of the reagent solution were sealed in m1. glass ampoules.

Reducing reagent solution-Twenty gm. of potassium cyanide, 40 gm. of ammonium citrate, and 200 gm. of anhydrous sodium sullite ywere dissolved in about l liter of water. This solution is diluted to 1.3 liters, and then mixed with 1.2 liters of concentrated aqueous ammonium hydroxide. Portions of 25 ml. of this mixture were sealed in glass ampoules. Deleading, to remove any trace quantities of lead which might be introduced as impurities in the reagents, can be provided according to known methods (see Snyder et al., Analytical Chemistry 20, 772 (1948)). However, this ordinarily is not required, and the presence of minute quantities of lead in the reagents is compensated for, in actual operations, by a constant correction factor determined by a blank determination.

Dithizone solution-About 40 rngs. of dithizone were dissolved in 20 mls. of chloroform. Portions of this solution, amounting to 0.20 ml. were introduced into screw cap bottles, and the chloroform removed by vacuum evaporation, leaving relatively precise quantities of dithizone crystals in each small bottle which was then sealed.

Partition solution- 500 mls. of absolute methanol, 360 mls. of carbon tetrachloride, and 90 mls. of chloroform were mixed together and 5 mls. portions of these were sealed in glass ampoules.

In a specific illustration of application of the method, one cubic foot samples of air, having established quantities of tetramethyllead vaporized therein, were drawn through individual iodine crystal beds, held in place by fibrous glass layers as already described. The iodine crystal layer was then Iwashed with about 5 mls. of the acidic potassium iodide solution in increments of one or two mls. The treatment with the potassium iodide wasr spread over about 4 or 5 minutes to assure full conversion of the lead to lead iodide, as it has been found that the initial compound formed is dimethyl lead di-iodide, which thereafter quickly decomposes to lead iodide. The reaction with the iodide solution was followed lwith a rinse of about 20 mls. of distilled Water.

The liquid was introduced to a color comparator tube,

having a 13 x 13 mm. square end, such as is used in a Hellige color comparator.

One of the ampoules containing 25 mls. of the reducing reagent solution ywas then opened and about 5 mls. were poured into one of the specimen bottles having solid dithizone crystals, which dissolved therein immediately, forming an ammonium dithizonate solution. The residual 20 mls. reagent solution were poured directly into the color comparator tube and mixed well for prompt reduction of all free iodine. The last step was the addition of the dithizonate solution, and 5 mls. of the alcoholic carbon tetrachloride-chloroform solution. The mixture was shaken vigorously to extract the lead dithizonate into the chlorinated hydrocarbon phase. The mixture was then allowed to separate and the carbon tetrachloridechlorofrom phase, containing lead dithizonate dissolved therein and having a red color, stratified at the bottom of the color comparator tube in the square section. The color comparator tube Iwas then inserted into the Hellige color comparator for matching with a standard color portion of a colored disc. The volume of chlorohydrocarbon solvent provided assures that the number appearing on the Hellige color comparator disc equal the micrograms of lead per cubic foot of air.

A series of determinations following the above procedure, were carried out, the first being a blank run. The following results were obtained:

The foregoing shows the excellent degree of agreement obtained between the analyzed results and the known composition. The indicated accuracy is quite satisfactory for hygienic observations of tank atmospheres or the like, prior to repair or cleaning operations. When a higher degree of precision is needed, the solution of lead dithizonate can be more accurately analyzed by an instrumental examination, i.e., by a spectrophotometer.

IIn addition to the above demonstrated eiiiciency of the improved sampling and analysis technique applied to determination of tetramethyllead in gas, comparable efiiciencies have been found when the lead compound is tetraethyllead, tetravinyllead, tetra isoamyllead and tetraphenyllead.

A lengthy series of tests has shown unusual e'iciency of scrubbing for a variety of organolead compounds, Whereas attempts to scrub the same component from gas with an aqueous iodine solution have been only very disappointing. Typical comparative eiciencies, at sampling rates of 0.75 cu. ft. per minute, are:

The sampling rate should not be too great. Generally, a rate of 0.5 to 1.5 cubic feet per minute is quite satiscfactory. When the sample is known to contain higher quantities of lead compounds, or when appreciably larger samples are desired for some purposes, the quantity of iodine crystals, used in the first and important contacting step, can be appreciably increased. Similarly, the physical dimensions of the chamber in which the iodine crystals are contained, can be increased in cross sectional area to maintain approximately the same linear rate of liow of the gas sample as indicated above.

The use of fibrous, inactive material to position the iodine crystals in place in the contacting chamber, is necessary and important. Clearly, a support means is essential in the rst instance, and a bed of fibrous material is necessary to assure that any component, resulting from reaction of a lead component and the iodine crystals, which itself might be volatile to some extent, would be entrapped in the fibrous material. Instead of glass wool, mineral Wool, or entirely inert metallic fibrous material or wool can be substituted. However, for all practical purposes, glass bers or glass wool, being so readily available and economical, is preferred.

As already explained, a signicant factor in the eliiciency of the method is the use of a partition solution including two types of components, viz., a water insoluble constituent, and a 4water soluble constituent. As illustrated above, typical and preferred examples of these co-nstituents are the mixture of carbon tetrachloride and chloroform, as the iirst constituent and absolute methanol as the second constituent. It will be apparent that, instead ofthe precise proportions as illustrated by the above example, the amount of carbon tetrachloride-chloroform constituent can be doubled in instances where the standard volume of a sample specimen is doubled, and the final determination will still be in terms of micrograms 0f lead per cubic foot.

Other variations in the general procedure will be readily apparent to those skilled in the art.

What is claimed is:

In the collection and determination of trace quantities of tetrahydrocarbon lead impurities in gaseous fluids, said method comprising contacting a sample of gas with an iodine reagent to form a lead iodide reaction product, and thereafter determining the thus formed lead iodide by the dithizone method, the improvement consisting of passing the gas sample through a tertiary layer contacting system, the layers thereof consisting of a first and third layer of inert brous material and a middle layer of anhydrous,

potassium iodide solution through said tertiary layer system, thereby establishing an aqueous solution having lead iodide and elemental iodine therein and thereafter analyz ing said aqueous solution by the dithizone method.

References Cited in the le of this patent UNITED STATES PATENTS 2,366,953 Beatty lan. 9, 1945 OTHER REFERENCES Snyder et al.: AnaL Chem. 20, 772-776 (1948). Mellor: Comprehensive Treatise on Inorganic and Theoretical Chemistry, vol. 7, page 757, Longmans, Green &

elemental iodine crystals, and thereafter passing aqueous 15 Co., New York, 1927. 

1. IN THE COLLECTION AND DETERMINATION OF TRACE QUANTITIES OF TETRAHYDROCARBON LEAD IMPURITIES IN GASEOUS FLUIDS, SAID METHOD COMPRISING CONTACTING A SAMPLE OF GAS WITH AN IODINE REAGENT TO FORM A LEAD IODINE REACTION PRODUCT, AND THEREAFTER DETERMINING THE THUS FORMED LEAD IODIDE BY THE DITHIZONE METHOD, THE IMPROVEMENT CONSISTING OF PASSING THE GAS SAMPLE THROUGH A TERTIARY LAYER CONTACTING SYSTEM, THE LAYERS THEREOF CONSISTING OF A FIRST AND THIRD LAYER OF INERT FIBROUS MATERIAL AND A MIDDLE LAYER OF ANHYDROUS, ELEMENTAL IODINE CRYSTALS, AND THEREAFTER PASSING AQUEOUS POTASSIUM IODIDE SOLUTION THROUGH SAID TERTIARY LAYER SYSTEM, THEREBY ESTABLISHING AN AQUEOUS SOLUTION HAVING LEAS IODIDE AND ELEMENTAL IODINE THEREIN AND THEREAFTER ANALYZING SAID AQUEOUS SOLUTION BY THE DITHIZONE METHOD. 